Inside Salt Lake City's dreary, dangerous smog dome

In Salt Lake City, Utah, residents call winter "inversion season" for its tendency to bring on a strange weather condition: Snowstorms invert the usual trend of air getting colder as the altitude gets higher by cooling the ground, which leaves behind a layer of warm air high above the metropolis. An inversion creates a sort of atmospheric roof while the mountains surrounding the city act like walls, trapping pollution in place until another storm blows it away. The resulting "smog dome" lasts four days on average, but some have lingered almost three weeks. These periods aren't just dreary; they're dangerous. Here's a peek into the dome.

A Dome Is Born

Normally, higher altitudes have colder air, but a couple of scenarios can set a warm-air "lid" over an area. Salt Lake City's inversions usually follow snowstorms. Other locales more often get nighttime radiation inversions, when Earth gives off more heat than it receives from the sun.

Peak Problems

Smog domes can happen anywhere but occur more often in cities—such as Boise, Idaho, and Los Angeles—that lie near mountain ranges, which trap cold air in valleys. Domes also tend to hit areas that get heavy snowfall, which makes air near the ground cooler than what's above.

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Smog in Salt Lake City

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Smog in Salt Lake City

DRAPER, UT - JANUARY 31: Evening lights shine through as a temperature inversion traps and fills the Salt Lake valley with thick smog as seen from Traverse Mountain on January 31, 2017 in Draper, Utah. Severe inversions have plagued Utah's large cities for many years. (Photo by George Frey/Getty Images)

DRAPER, UT - JANUARY 31: Evening lights shine through as a temperature inversion traps and fills the Salt Lake valley with thick smog as seen from Traverse Mountain on January 31, 2017 in Draper, Utah. Severe inversions have plagued Utah's large cities for many years. (Photo by George Frey/Getty Images)

DRAPER, UT - JANUARY 31: Evening lights shine through as a temperature inversion traps and fills the Salt Lake valley with thick smog as seen from Traverse Mountain on January 31, 2017 in Draper, Utah. Severe inversions have plagued Utah's large cities for many years. (Photo by George Frey/Getty Images)

LINDON, UT - JANUARY 31: Cars and trucks make their way down I-15 through Utah valley filled with thick smog on January 31, 2017 in Lindon, Utah. Severe inversions have plagued Utah's large cities for many years.(Photo by George Frey/Getty Images)

ALPINE, UT - JANUARY 31: A temperature inversion traps and fills Utah valley with thick smog as seen from Traverse Mountain on January 31, 2017 in Alpine, Utah. Severe inversions have plagued Utah's large cities for many years.(Photo by George Frey/Getty Images)

SALT LAKE, UT - JANUARY 31: With the Utah State Capitol in the foreground, a temperature inversion traps and fills downtown Salt Lake City with thick smog on January 31, 2017 in Salt Lake City, Utah. Severe inversions have plagued Utah's large cities for many years. (Photo by George Frey/Getty Images)

SALT LAKE, UT - JANUARY 31: A temperature inversion traps and fills downtown Salt Lake City with thick smog on January 31, 2017 in Salt Lake City, Utah. Severe inversions have plagued Utah's large cities for many years. (Photo by George Frey/Getty Images)

ALPINE, UT - JANUARY 31: A temperature inversion traps and fills Utah valley with thick smog from a temperature inversion as seen from Traverse Mountain on January 31, 2017 in Alpine, Utah. Severe inversions have plagued Utah's large cities for many years.(Photo by George Frey/Getty Images)

DRAPER, UT - JANUARY 31: A temperature inversion traps and fills the Salt Lake valley with thick smog as seen from Traverse Mountain on January 31, 2017 in Draper, Utah. Severe inversions have plagued Utah's large cities for many years. (Photo by George Frey/Getty Images)

A small housing development rises above the smog-filled valley of Salt Lake City in Utah, U.S., on Monday, Jan. 11, 2010. In the Salt Lake City area during the winter, atmospheric inversions can trap pollutants such as ozone and carbon monoxide near ground level, producing a dense smoglike cover. Photographer: George Frey/Bloomberg via Getty Images

The Utah State Capitol, left, stands past smog from a winter temperature inversion in Salt Lake City, Utah, U.S., on Wednesday, Feb. 10, 2016. With warmer air above and cooler in the valleys, air pollutants get trapped in the valleys by the inversion that acts like a lid, causing sever air pollution. Photographer: George Frey/Bloomberg via Getty Images

Cars make their way along a highway in heavy smog south of downtown Salt Lake City in Utah, U.S., on Monday, Jan. 11, 2010. In the Salt Lake City area during the winter, atmospheric inversions can trap pollutants such as ozone and carbon monoxide near ground level, producing a dense smoglike cover. Photographer: George Frey/Bloomberg via Getty Images

Houses sit above the smog-filled valley of Salt Lake City, looking from the Sun Crest development in Draper, Utah, U.S., on Monday, Jan. 11, 2010. In the Salt Lake City area during the winter, atmospheric inversions can trap pollutants such as ozone and carbon monoxide near ground level, producing a dense smoglike cover. Photographer: George Frey/Bloomberg via Getty Images

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Something in the Air

Inversions trap fine particles known as PM2.5, produced by car engines, bonfires, and industrial emissions. The buildup of these and other pollutants, such as ozone, can reduce visibility, increase the acidity of water, and deplete soil nutrients.

Bated Breath

Inhaling PM2.5 can exacerbate health issues such as asthma and congestive heart failure: Whenever Salt Lake City has an inversion, emergency-room visits spike. And the longer it lasts, the sicker people get. If concentrations rise too high, students stay indoors and adults telecommute.

Dome-ination

Humans can't stop inversions. But we can cut down on dome-filling emissions. Salt Lake aims to increase car-efficiency standards and reduce traffic with better public-transportation options. Officials hope that small measures like these, taken together, will lighten the dome by 2040.

This article originally appeared in the July-August 2017 issue of Popular Science, under the title "Inside a Smog Dome."